


Prying Eyes

by Anonymonimus



Category: Undertale (Video Game)
Genre: Accidental Voyeurism, Alternate Universe, Alternate Universe - Underswap, Anal Sex, Doggy Style, Dom Sans, Ecto-Genitalia (Undertale), Ecto-Penis (Undertale), Ecto-Vagina (Undertale), Ectobiology, Exhibitionism, Friends With Benefits, Gay Sex, Gen, Gratuitous Smut, M/M, Masturbation, Missionary Position, Multiple Orgasms, Platonic Relationships, Platonic Sex, Rough Sex, Sans-centric, Self-cest, Sex, Shameless Smut, Skeleton Boyfriends, Skeleton Sex (Undertale), Skeletons, Smut, Sub Sans, Underswap Sans, Voyeurism
Language: English
Status: Completed
Published: 2017-01-28
Updated: 2017-01-28
Packaged: 2018-09-20 09:51:09
Rating: Explicit
Warnings: Creator Chose Not To Use Archive Warnings, No Archive Warnings Apply
Chapters: 1
Words: 2,845
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/9485855
Author URL: https://archiveofourown.org/users/Anonymonimus/pseuds/Anonymonimus
Summary: Blue wants to prove he can top.





	

**Author's Note:**

> **The tumblr audience may realise that I'm really not taking these requests in order. I do which ever one I feel like doing next. I'll get to all of them at some point or another lol**
> 
> **I want to just warn that I'm so fucking tired right now and I'm definitely going to sleep once I finish posting this. I tried proof reading this story on my own but I'm so fucking tired there might be gibberish here and there. Please warn me if you see a sentence that makes you go "what the fuck this is not the English language" so I can go fix it when I have all of my usual brain capacity.**
> 
> **Also, for reference:**
> 
> **Blue = US Sans  
>  Sans = UT Sans  
> Red = UF Sans  
> Papyrus = UT Papyrus**
> 
> **Now, ENJOY!**

Blue was rather annoyed. He had met with Red earlier and had had a conversation that had severely frustrated him. He couldn't quite remember how they had found themselves discussing the topic in question, but Red had essentially declared he would never be able to top anyone due to his 'innocent nature'. No matter how Blue had tried to argue about how no one in a relationship found themselves truly cemented as either bottom or top, the edgier alternate version of himself had refused to listen to reason. He had blocked his non-existing ears and shouted loudly in a childish manner to drown out his voice. Needless to say, Blue had abandoned his 'friend' for the far more pleasant company of his other alternate: Sans.

He paced around Sans's room grinding his teeth together as he contemplated just how angering his time with Red had been. The other skeleton sat on his mattress with a look of exasperation on his face. It was obvious that he was trying to be both supportive and patient as he quietly listened to Blue grumbling to himself. He must have understood how the former needed to vent despite not knowing what the source of his irritation was.

"so…" Sans eventually said, "are we going to keep doing…" he paused, searching for words and twirling his wrists until eventually gesturing his general surroundings, "this?"

"uurgh!" Blue growled, gripping his head which he then threw back, "i'm sorry, i know i'm being rude!" he exclaimed, "i'm just so… _mad_!"

"hey, no worries." Sans was quick to say and raise his hands defensively. "i get you're upset but, you know, we could talk about it."

Blue gasped. He was absolutely right. In his frustration, he hadn't considered the option. "you're right!" He agreed and jumped onto the free space next to Sans. The latter seemed off put by his enthusiasm. Blue was aware his energy sometimes daunted the people he was around. Oh well. He sucked in a deep breath and suddenly began ranting, speaking so rapidly it was difficult to distinguish the words from one another.

"whoa, uh, slow down buddy." Sans implored, "i have no idea what you just said."

"do you think i could never top someone!?" Blue yelled, grabbing Sans by the shoulders.

"…what?"

"i mean sexually!" Blue specified.

Sans blushed a bit and visibly began sweating nervously. "…what?"

Blue growled and flopped onto his back, lifting his legs to rest them on Sans's lap. "i was hanging out with red earlier," He explained more calmly, "we started talking about sex and he said i could never top because of my 'innocent nature'." Blue made sure to give Red a ridiculous voice when quoting him. "but that's not how it works! people switch between being the bottom or the top in sex all the time!"

"yeah, you're right." Sans agreed.

"right!?" Blue shouted, throwing his hands up.

"but," Sans then continued and Blue propped himself up on his elbows to look at his counterpart, "i can't really imagine that applying to you. red kind of has a point."

"sans!" Blue whined and allowed himself to fall back onto the mattress in exasperation, "you're supposed to be on _my_ side! you're supposed to be sensible and smart!"

"try seeing it from our point of views." Sans offered, "you're the cutest thing in the multiverse. i really can't see anyone letting you top them. it's the same thing for my pap. you two are the same."

Blue snorted. Sans had probably uttered the most ignorant thing he had ever thus far heard. If only his alternate knew all the dirty things his brother wanted to do to him. Papyrus would never let _him_ top him – at least, not until he finished checking off his list of perverted fantasies. But it wasn't his place to out his friend so Blue kept his mouth shut and opted to simply roll his eyes. It seemed that everyone who wasn't he or Papyrus had a skewed perception of them. It was both annoying and wrong.

"i could totally top someone." Blue pouted.

"not with that face." Sans teased and laughed.

Blue growled to himself. He had come to Sans because Red was being unbearable and he thought the former would be better, more understanding company. Clearly, he was wrong. He pulled his legs off Sans and curled into a fetal position then rolling onto his side as he continued to pout. "yes i could." He insisted.

Sans continued to laugh at the adamant remark. Finally brought to his wit's end, without thinking, Blue abruptly lunged at his counterpart and pushed and pinned him down. He quickly seized the other skeleton's wrists and restrained them above his head. He felt very powerful in his given position and Sans's astonished expression filled him with a certain smugness.

"i could top." Blue repeated, "i could specifically top you."

Though he wasn't quite sure Papyrus would approve. Blue wouldn't be able to benefit from the excuse that he didn't know, especially after the taller skeleton confessed his feelings to him a while ago. They also routinely discussed Sans so saying he had forgotten was also out of the equation.

"haha." Sans laughed stiffly, "very funny. you've made your point."

"have i?" Blue grinned. "aren't i too innocent to have made my point?"

Sans's blush grew, "my…my brother's downstairs." He said quietly.

Now it was Blue's turn to be astonished. "wait…you would actually let me?" He asked.

Sans flinched and nervously glanced away. "i-i mean…uh…no…?" he stammered.

"no! it's too late! you already said yes!" Blue shouted victoriously. Sans frowned in return. "hey, i'm not going to force myself on you. i'm just saying you admitted that you would let me top you if your brother wasn't here."

"don't flatter yourself." Sans huffed and rolled his eyes.

"what? would you let _anyone_ top you?" Blue teased.

"yeah." Sans shrugged as though it wasn't a big deal. "it's like you said: people switch roles all the time."

"that's even more confirmation that you would let me top you!" Blue cheered enthusiastically.

Sans sighed and rolled his eyes again. "oh my god." He grumbled.

Blue laughed proudly. If he could make Sans change his mind on how he perceived him, then surely he could do the same for Red. He couldn't wait to make the latter eat his words.

"so can you get off me now?" Sans asked.

"do i have to?" Blue pouted but then grinned, "wouldn't you rather i engage in the dance with no pants?"

Sans snickered at his phrasing. "any boners i could have had are now completely dead." He said, "besides, like i said, my brother's home."

Blue sighed and released Sans's hands. He sat back still straddling his hips. "why is that a problem?"

As Papyrus's friend, Blue probably shouldn't be considering actually having sex with Sans. However, his desires were getting the better of him. He always wondered what it would feel like to do anything sexual with one of his counterparts. In the end, wouldn't whatever they did be classified as masturbation? Was it such a big issue? Could it even be considered cheating?

Sans blushed and glanced away. "what if he comes upstairs and catches us?"

Blue glanced at the door. It was mostly closed with only the barest crack. Still, there was no way to see the hall with such little space, at least from their position in the room. "if we're quiet, we'll be fine, won't we?" he shrugged. "and isn't he cooking downstairs? even if we made _some_ sounds, the ruckus from his actions would muffle us."

Sans was visibly toying with the idea. Due to essentially being the same person, it was easy for Blue to identify the other's body language. The way his alternate-self nervously glanced at the door and fiddled with his hands betrayed just how much he actually wanted to indulge in what Blue was suggesting, but the fear of being caught was making him hesitate. However, 'fear' wasn't quite right. Blue didn't see the usual tells of when he really felt fear or anxiety. And that was when he realised Sans wasn't quite as afraid as he was excited.

"but…" Blue started slowly, "maybe that's what you want." Sans flinched and blushed darkly, "maybe you get off on the idea of being caught while getting fucked?"

Sans glanced away, "it's…it's not…" he mumbled pointlessly.

Blue grinned. "how perverted of you." He teased. Though, if he were honest, he was getting turned on with how charmed Sans seemed at the thrill of being caught. That being said, he may also have found something arousing in going behind his friend's back and doing what he was about to do – no matter how bad he already knew it was. Maybe Papyrus would forgive him.

Blue moved off his counterpart and proceeded to manhandle him. He pushed Sans on his hands and knees facing the door while he moved behind and leaned over him. "how about we do it like this?" he suggested, "you can keep a watch on the door while i fuck you. sound fair?"

"w-what?" Sans stammered nervously, "if he comes in—"

"he'll have a pretty view of your face." Blue cut in smoothly. "admittedly, it makes me a bit jealous, but i'll get over it." he grinned as he paused but then continued in a low tone with: "but i'm sure you'd like that. you probably want to get caught, don't you?"

Sans tensed but said nothing. He eventually sighed and glanced back. "this 'top' Blue character of yours is really a dick." He commented bluntly, breaking the mood.

"i know, but he's quite fun to portray." Blue beamed. "besides, i don't think my usual self is suitable for a domineering top."

"you're right, it's not." Sans agreed. He then lowered his torso onto his mattress while reaching around with one hand to pull down his shorts and expose himself to Blue. "well then, let's get on with it."

Blue blinked and blushed at the sight of the cunt his counterpart had conjured up for him. He shook his head and smiled brightly, declaring: "with pleasure!"

He took Sans by the pelvis and adjusted his position. The latter made a cute sound he tried to keep contained, but it hadn't managed to escape Blue's vigilance. He planned to make the other utter many more sounds in the very near future anyhow. Satisfied, he focused his magic to conjure an ecto cock and lowered his pants to free it from its confines. He had only topped once before with his own brother. Though he had been told he had been excellent, Blue recognised how easily his brother could have lied to him for the sake of his self-confidence. His Papyrus was just cool that way. Regardless, he really hoped he wouldn't disappoint Sans – though he kept his concerns to himself.

Blue took his cock and guided it to the cunt the other had made. He considered simply inserting it but decided to tease a little. He rubbed his length against the ecto pussy making Sans quiver and whimper in delight and anticipation. It didn't take long before he had gotten enough and began begging him to put his prick inside.

"please just hurry up." Sans whined.

Blue was aroused by the plea and felt all the more smug when remembering how the skeleton in question had once affirmed he wouldn't be able to top anyone. Now, not only was he doing so, but he was getting Sans to beg for him. Regardless, he didn't keep him waiting. He slowly pushed in his erection earning a startled moan and whimper from Sans. His cunt felt so good and warm. It clung to his cock and throbbed around it, begging it to move and ravage it.

"keep your eyes on the door and your voice down." Blue instructed, "or do neither if you would rather make a little show for papyrus."

And before Sans could respond with denial or some poorly thought up lie, Blue began to thrust. His first few movements were experimental. He wasn't moving quite as fast or as hard as he wanted just yet; more curious to see how Sans reacted. Unsurprisingly, the skeleton was rather loud despite his tamed behaviour. He was either naturally noisy during sex or he was trying to draw Papyrus upstairs. Though the latter scenario would certainly have negative repercussions on their friendship, Blue decided that, in this very moment, he didn't care.

With his experimenting done, Blue began to get more aggressive with his ministrations. He plowed into Sans rapidly and harshly without the courtesy of a gradual buildup. Their members slapped together wetly, deafening Blue to practically everything but the delicious sounds that poured out of his counterpart's mouth. For someone with a raspy and low voice, Sans made the most heavenly of moans. It truly was music to Blue and he could hardly handle it. The cries were smooth and high, filled to the brim with passion and pleasure. There was an addicting nature to them; one that encouraged him to keep vigorously plowing into Sans for more.

He was so focused on his heavenly moans he almost didn't catch a glimpse of Papyrus spying from the crack. He was hit with a pang of guilt but then, knowing his friend, the taller skeleton was probably very aroused by the sight. He had mentioned once before wanting to have a threesome with Sans and maybe Blue himself or another Sans counterpart. Regardless, Blue wasn't going to stop now – not that he could.

" _g-god!_ " Sans cried, clutching at his mattress desperately. "b-blue! _ahhn!_ this feels so good!"

"you feel fantastic, Sans." Blue grunted as he snapped his hips, giving a particularly deep thrust. "your pussy is squeezing me so perfectly like it wants my cum."

" _yes! hnn! yes please!_ " Sans begged, shyly moving to meet the other's thrusts. "i-i want your cum inside me!"

"i'll give it to you, i promise." Blue groaned.

His thrusts picked up speed and Sans moaned and shouted his bliss all the louder. It seemed like he truly didn't care if Papyrus was hearing him – almost like he had forgotten about him. Said behaviour told Blue he likely hadn't noticed the prying eyes hidden behind the door. Though the thought quickly escaped his mind as the pleasure was quickly driving him to the edge. It would only take a little more of him to cum, but before he could get there, Sans tightened drastically around him and came with a loud, heavenly cry.

Blue reluctantly stopped his movements and allowed the other skeleton to collapse in satisfaction. He seemed dazed by the strength of his ejaculation, mindlessly staring off into space as he breathed heavily and whimpered and quivered from steadily fading sparks of ecstasy. Needless to say that Blue was fairly disappointed by how quickly this had finished.

"geez, i thought you would have a little more stamina than that." Blue whined and sat back. He glanced down at his boner regretfully. "i didn't even get the chance to cum."

To his surprise, Sans rolled on his back and parted his legs. "you…you can finish inside me…" He said.

Blue beamed excitedly at the offer. "wowie! okay!" He moved back in between Sans's legs and pushed his prick in the abused, twitching hole. The skeleton gave a loud cry and quivered at the penetration. "whoa…you're pretty sensitive now, hun?"

"a-a little…" Sans conceded with a quivering voice.

"guess you really want my cum." Blue grinned and gave a hard thrust that had Sans gasping and crying. "or maybe you just wanted to give papyrus an additional show…"

Sans looked shocked but could do nothing but cry and shout in overwhelming bliss as Blue set a punishing pace to finish himself off. That said, the latter skeleton still noticed how his eyes routinely shifted towards the door probably in the vain attempt to catch a glimpse of Papyrus. He also noticed how Sans seemed all the more excited knowing there was an audience of one witnessing his vulgar disposition.

" _mmf! blue!_ " He moaned lewdly, throwing his arms haphazardly over his head as he abandoned himself to lust and pleasure. " _fuck me! oh god, please fuck me!_ "

Blue continued to plow into the other skeleton, drinking in the lecherous things that fell out of his mouth as he seemingly lost his mind. A handful of thrusts before he reached his climax, Sans came again but this time squirting as his orgasm shook his body. Several thrusts after, Blue finally filled his cunt with his seed and collapsed onto him.

"i was really wrong about you not being suitable for a top." Sans breathed as he came down from the sex afterglow.

"of course! the magnificent sans can do anything!" Blue boasted.

"but it looks like i'm also not the only one who likes to get off with an audience." He teasingly added.

"guilty." Blue admitted.

**Author's Note:**

> **I imagine that the Sanses established that they needed to use nicknames for one another because calling everyone "Sans" would just be confusing so only one of them could still use the name. I reckon the way they decided who could still be called "Sans" was by drawing the short straw. I like to think UF Sans picked the short straw but was being so obnoxious about getting to use his name while the other two were stuck with nicknames that US Sans and UT Sans decided UT Sans would use "Sans" instead and UF Sans would be stuck with a nickname as punishment.**


End file.
